package com.leedsoft.common;

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

/**
 * An LRU cache, based on <code>LinkedHashMap</code>.
 * <p>
 * <p>
 * This cache has a fixed maximum number of elements (<code>cacheSize</code>).
 * If the cache is full and another entry is added, the LRU (least recently used) entry is dropped.
 * <p>
 * <p>
 * This class is thread-safe. All methods of this class are synchronized.
 * <p>
 * <p>
 * Author: Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>
 * Multi-licensed: EPL / LGPL / GPL / AL / BSD.
 */
public class LruCache<K, V> {

    private static final float hashTableLoadFactor = 0.75f;

    private LinkedHashMap<K, V> map;
    private int cacheSize;

    /**
     * Creates a new LRU cache.
     *
     * @param cacheSize the maximum number of entries that will be kept in this cache.
     */
    public LruCache(int cacheSize) {
        this.cacheSize = cacheSize;
        int hashTableCapacity = (int) Math.ceil(cacheSize / hashTableLoadFactor) + 1;
        map = new LinkedHashMap<K, V>(hashTableCapacity, hashTableLoadFactor, true) {
            // (an anonymous inner class)
            private static final long serialVersionUID = 1;

            @Override
            protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
                return size() > LruCache.this.cacheSize;
            }
        };
    }

    public LruCache() {
        this(30);
    }

    /**
     * Retrieves an entry from the cache.<br>
     * The retrieved entry becomes the MRU (most recently used) entry.
     *
     * @param key the key whose associated value is to be returned.
     * @return the value associated to this key, or null if no value with this key exists in the cache.
     */
    public synchronized V get(K key) {
        return map.get(key);
    }

    /**
     * Adds an entry to this cache.
     * The new entry becomes the MRU (most recently used) entry.
     * If an entry with the specified key already exists in the cache, it is replaced by the new entry.
     * If the cache is full, the LRU (least recently used) entry is removed from the cache.
     *
     * @param key   the key with which the specified value is to be associated.
     * @param value a value to be associated with the specified key.
     */
    public synchronized void put(K key, V value) {
        map.put(key, value);
    }

    public synchronized void clear() {
        map.clear();
    }

    public synchronized int size() {
        return map.size();
    }

    /**
     * Returns a order list of all cache entries.
     * list的0是缓存队列的queue尾， list的列尾是缓存队列的queue头。
     */
    public synchronized List<Map.Entry<K, V>> getAll() {
        return new ArrayList<>(map.entrySet());
    }

}
